Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
Protein Synthesis and Its Regulation
Early discoveries laid the foundation for research into protein biosynthesis

The foundation of our current understanding of METABOLISM/35.html">Protein Biosynthesis was laid by three major discoveries in the 1950s. In the early 1950s, Paul Zamecnik and his colleagues at the Massachusetts General Hospital raised the question: at which cellular site are Proteins synthesized? In search of an answer, they administered radioactive Amino Acids to rats.

At various time intervals following the injection, they excised the Liver, homogenized it, fractionated it by centrifugation (Sec. 13.16), and then examined the resulting subcellular fractions for the presence of radioactive protein. When hours or days had elapsed after the administration of labeled amino acids, the labeled proteins were found in all intracellular fractions. However, when the liver was removed and fractionated just a few minutes after the injection of labeled amino acids, newly synthesized labeled protein was detected exclusively in the fraction containing small ribonucleoprotein particles. Based on these findings, it was concluded that the site of Protein Synthesis from amino acids is the ribonucleoprotein particles, which had previously been observed in animal Tissues using the Electron microscope (Sec. 2.17); they were later named Ribosomes (Fig. 29-1).

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Fig. 29-1. A region of a pancreatic Cell showing ribosomes attached to the outer surface of The Endoplasmic reticulum.

The second discovery was made by Mahlon Hoagland and Paul Zamecnik, who found that incubating amino acids with ATP and the cytosolic fraction of liver Cells leads to Amino Acid Activation. In this enzymatic process, the amino acids became attached to a specific heat-stable soluble RNA later designated as Transfer RNA (tRNA).

The third of the aforementioned major discoveries belongs to Francis Crick, who pondered how the Genetic information encoded in Nucleic Acids via a four-letter alphabet is translated into the twenty-letter language of proteins. Crick concluded that tRNAs must act as adapters in this process. One part of the tRNA molecule can bind to a specific amino acid, while another part recognizes a short nucleotide sequence in the mRNA that encodes that amino acid (Fig. 29-2).

It was precisely these three discoveries that soon led to the elucidation of the principal stages of Protein Biosynthesis and, ultimately, to the cracking of The Genetic Code for amino acids.

Fig. 29-2. Crick's hypothesis on the adaptor function of tRNA. Today we know that The amino acid is covalently linked to the tRNA, and a specific nucleotide triplet in another region of the tRNA molecule "recognizes" the coding triplet of the mRNA through hydrogen bonding between complementary bases.